1. Field of the Invention
The invention is related to electrosurgical instruments and techniques for precision application of energy to tissue, and more particularly to a system for thin layer ablation. More in particular, the working surface of a probe carries (i) delivery means for introducing and capturing a gas volume in a microchannel structure at an interface between the probe and the targeted site, (ii) energy delivery means for creating an intense electrical field of micron-dimensioned gas volumes to controllably apply intense energy to the targeted site to cause volumetric tissue removal, and (iii) vibration means for assisting in the creation of micron-scale gas volumes or bubbles.
2. Description of the Related Art
Various electromagnetic and acoustic energy delivery sources have been investigated for surgical tissue ablation or removal, including radiofrequency (Rf) current flow within tissue, high intensity focused ultrasound (HIFU) tissue interactions and microwave energy absorption in tissue. In general, at high intensities, the above listed energy sources generate thermal effects that can vaporize tissue as the means of tissue ablation or removal. In other words, the energy sources elevate the temperature of water in intra- and extracellular spaces to above 100° C. thereby explosively vaporizing water to damage or destroy the tissue. The drawback to such purely thermally-mediated ablations is significant collateral damage to tissue volumes adjacent to the targeted site. While in many surgical fields, the above-described collateral thermal damage may be acceptable, in fields such where thin layer ablations are required, such as skin resurfacing, ophthalmology, neurology, and interventional cardiology, there is a need to prevent, or limit, any such collateral damage.
Lasers for Use in Tissue Ablation.
Various laser systems have been developed for tissue ablation. The conventional long-pulse laser systems outside the UV range, wherein long-pulse is defined as a system operating in a range of 10's of nanoseconds to microseconds in pulse duration, have been found to be inefficient in volumetric tissue removal without causing extensive collateral damage. In a the conventional long-pulse laser system (e.g., Nd:YAG, Er:YAG, IR lasers), the photonic energy delivered to a targeted site does not directly disrupt the molecular integrity of surface layers of the site, but rather the energy is transferred into surrounding tissue volumes as photothermal energy, or photomechanical energy. These collateral effects propagate through surrounding tissues as heat, and perhaps mechanical shock waves, which manifest themselves as undesirable collateral damage. More specifically, the generally accepted model of volumetric ablation or removal with lasers having a pulse longer than tens or hundreds of picoseconds is described as follows: The energy absorption is chromophore dependent (and/or scattering dependent), and the energy transfer involves the heating of conduction band electrons by the incident beam of coherent photons which is followed by transfer of thermal energy to the structure's lattice. Ablation or damage occurs by conventional heat deposition resulting in vaporization, melting, or fracture of the structure. The rate of volumetric structure removal depends on thermal conduction through the structure lattice and its thermodynamic properties (heat capacity, heat of vaporization, heat of fusion, etc.). Thus, the minimum energy requirements to cause an ablation effect in the structure's properties may be defined by a threshold of incident laser energy per unit of structure volume at the target site, which threshold is directly dependent on pulse duration. It has been found that ablation thresholds generally require relatively long pulse durations, which in turn are the source of undesirable collateral photothermal (or photomechanical) damage.
In certain tissue ablation fields (e.g., corneal ablation in ophthalmology), excimer lasers have been developed that emit high intensity pulses of ultraviolet (UV) light, typically with pulse durations in the 1 ns to 100 ns range. The short wavelengths, as well as sequenced nanosecond pulse regimes, define a substantially non-thermally mediated form of tissue ablation. Short wavelength UV photons are highly energetic and when radiated onto biological tissue can directly break the chemical bonds in surface layer molecules of the tissue. As a consequence, UV excimer lasers can vaporize or breakdown a surface tissue layer with minimal thermal energy transfer to underlying (or adjacent) tissue volumes. The by-product of the breakdown is predominantly a gas that is ejected away from the surface on which the energy was deposited, thus generally leaving the subsurface layer free from substantial collateral thermal damage. Tissue ablation with UV irradiation with can be controlled depth-wise since biological tissues exhibit strong absorption in the UV region of the electromagnetic spectrum (e.g., at c. 1.93 μm). In biological tissue, UV radiation typically only penetrates to a depth of from about 0.25 μm to about 4.0 μm per pulse in the ns duration pulses described above. Thus, to ablate to a certain depth, the system uses a pre-determined number of pulses of ns energy delivery
While UV photonic energy delivery can reduce collateral damage in tissue ablation, there are numerous disadvantages that limit the applicability of UV lasers to biomedical procedures. First, UV photonic energy delivery cannot be easily delivered to a targeted site of a body structure in a fluid environment (e.g., thrombus in a blood vessel) since intervening fluid may absorb energy rather than the target site. For this reason, UV energy delivery is most useful for ablating tissue surfaces exposed to the atmosphere, such as a patient's cornea in a LASIK procedure. Second, the desired lack of collateral damage in UV ablation is known to occur when a single pulse of UV photonic energy irradiates a tissue surface. However, when the UV pulse repetition rate exceeds about 5 to 10 Hz, considerable photothermal collateral damage (as well as photomechanical collateral damage) has been observed. Thus, UV ablation generally may result in low volumetric removals of tissue surfaces per unit of time. Third, while UV photons carry sufficient energy to directly break chemical bonds in surface molecules of tissue, UV wavelengths also may be sufficiently energetic to promote mutagenic effects thus elevating concerns about the long-term health and health of the clinician and the patient.
Conventional Electrosurgical Ablation of Tissue Volumes.
Radiofrequency currents in tissue have been known for many years in the prior art for cutting a tissue mass, or for coagulating regions within a tissue mass. Conventional electrosurgical systems known in the art ablate tissue by applying an electrical field across the tissue to be treated. The actual energy-tissue interaction in Rf cutting is typically described in terms of a voltage differential that causes a spark or arc across a gap between an active electrode 2a and the targeted site (e.g., coupled to a return electrode 2b) as shown in FIG. 1A. In the prior art instrument of FIG. 1A, a high energy density capable of tissue cutting is created when the gap between the active electrode 2a and the tissue surface is occupied with an electrically non-conductive gas, or an electrically insulative liquid. FIG. 1A depicts a typical ablation modality in which electrode 2a is moved into contact with a liquid or moisture layer on an exposed tissue mass which vaporizes a plurality of random bubbles 3. The bubbles comprise insulative gas volumes and randomly can form a momentary insulative physical gap between the active electrode 2a and the tissue (coupled to return electrode 2b) resulting in a spark across the gap. Such an Rf spark created between the active electrode and the tissue will cause localized damage and ablation at the discharge conduction site at the surface of the tissue. In other words, the spark causes very high energy densities at the random location that the tissue interfaces the bubble 3 which in turn results in intense heat that disrupts and ablates a site on the tissue surface. In FIG. 1A, it can be seen that conductive paths 4 indicate the paths of current flow. The conventional electrosurgical ablation of FIG. 1A generally is achieved at frequencies ranging from 500 kHz to 2.5 MHz, with power levels ranging from 75 to 750 W. In such prior art tissue cutting with Rf currents, the current density rapidly decreases with distance from the exact energy deposition site on the tissue which is contacted by the spark. Still, the depth of tissue disruption and damage in such prior art electrosurgical cutting may range from about 0.3 mm. to as much as 3.5 mm. (see R. D. Tucker et al, Histologic characteristics of electrosurgical injuries, Journal Am. Assoc. Gynecol. Laparoscopy 4(2), pp. 201-207 1997.) The depth of tissue ablation depends on several variables, including (i) the conductivity of the tissue, (ii) the insulative characteristics of the media in the physical gap between the active electrode(s) and the tissue; (iii) the dimension of the physical gap between the electrode(s) and the tissue; (iv) the power setting and optional feedback control of the power level based upon electrical characteristics of the targeted tissue; (v) and the translation of the working end relative to the tissue.
Electrosurgical Ablation with the Coblator™ System.
A recently commercialized invention in the field of electrosurgical ablation was invented by Eggers et al and is a called Coblator™ system (see. e.g., disclosures of Eggers et al in U.S. Pat. Nos. 5,873,855; 5,888,198; 5,891,095; 6,024,733; 6,032,674; 6,066,134 and the companion patents cited therein). The Coblator™ system relies on the creation of a voltage difference between a plurality of closely spaced rod-like electrode elements 2a and a return electrode 2b (see FIG. 1B) wherein the working end carries both the active and return electrodes. The Coblator™ system differs from conventional electrosurgical devices in that the system introduces an electrically conductive fluid such as isotonic saline 5 into the physical gaps 6 between the closely spaced active electrodes, and between the electrode group and the targeted tissue. The system applies electrical energy with a frequency of about 100 kHz and a voltage of about 100 to 300 V.
The Coblator™ company promotional materials explain that at high voltage levels, the electrically conductive fluid 5 in the gaps 6 that intervene between the closely spaced active electrodes 2a is converted into an ionized vapor or plasma. As evidence of the character of such a plasma, studies have shown that a typical plasma has an orange glow. Spectroscopic analysis of Coblator emissions show an emission peak of around 590 nm which is characteristic of the sodium ionization peak (NaCl normal solution used as conductive fluid), with negligible emissions above 600 nm. The company promotional material claims that conventional electrosurgical ablation yields a continuous spectrum from 490 nm to 900 nm, peaking at around 700 nm.
The supposition underlying the Coblator™ is that the actual energy-tissue interaction produced by the system relates to charged particles in the plasma having sufficient energy to cause dissociation of molecular bonds within tissue structures that come into contact with the plasma. Based on this hypothesis, the accelerated charged particles have a very short range of travel, and the energy-tissue interaction is confined largely to thin surface layers. Further, the supposition is that the energy-tissue interaction is a “cold” process that does not rely on the thermal vaporization of intra- and extracellular fluids to ablate tissue. In this respect, the Coblator™ system has been described as producing an ablation that compares to that of an excimer laser, both of which produce similar ablation by-products—if relying on comparison of spectrographic emission peaks. The energy required to cause molecular break-down of common molecular bonds in tissue is believed to be in the range of 3.0 eV to 5.0 eV or more. Considering the amount of energy utilized by the Coblator™ system to initially and thereafter continually vaporize NaCl from within a saline solution, it raises the question whether the plasma volumes can sustainably provide the energy levels required for true molecular dissociation of compositions in tissue surfaces, as with an excimer laser.
Another hypothesis that explains the Coblator™ ablation process in more mundane. Referring to FIG. 1B, it can be seen that the Coblator™ working end traps conductive fluid 5 within the many gaps 6 between the closely spaces electrode elements. As electrical potential is increased at the electrodes 2a, random and dynamic conductive paths are created to the return electrode 2b from random discharge points on the active electrodes 2a. Such conductive paths essentially comprise a dynamic uncontrolled distribution of NaCl molecules in the fluid, that can be momentarily linked by high energy densities along the conductive path that will, in turn, vaporize such compositions. The result is a frothy environment of random expanding and collapsing bubbles 3 as shown in FIG. 1B. Since the random transient bubbles 5 may comprise a quasi-neutral gas or a substantially insulative gas—thereafter a spark or discharge path 4 can occur along some random routes between the active electrode 2a and the return electrode 2b and within the bubbles 3. As shown in FIG. 1B, when such random insulative bubbles and an electrical spark path 4 occurs with the tissue surface being intermediate to the active and return electrodes, a spark-type energy-tissue interaction will occur that delivers ablative energy to the tissue at any random location that a bubble 3 in the path 4 interfaces the tissue. While FIG. 1B shows two random spark-type events through a frothy gas bubble environment, if this hypothesis were accepted, an actual Coblator™ ablation comprises 1000's of such random and discrete spark-type events per second to cause tissue ablation. According to this hypothesis, the Coblator™ system would produce an energy-tissue interaction much like that of conventional electrosurgical ablation as depicted in FIG. 1A.
This hypothesis easily explains the two observations made by proponents of the Coblator™ that state (i) that spectroscopic analysis of energy delivery within a conductive fluid supports the theory of molecular dissociation, and (ii) that evidence of limited collateral thermal damage (so-called “cold” ablation) must be the result of a molecular dissociation process. First, the spectroscopic analysis of the Coblator™ ablation certainly would show emission peaks for vaporization of the sodium analytes in the fluid, which is a primary sink of the energy applied from the device—which is different from and predominates over any emission peaks from the tissue ablation. The tissue ablation, if the competing hypothesis of a spark-type interaction of FIG. 1B is accurate, would be characterized by energy-to-tissue events that produce mostly water vapor from the vaporization of intra- and extracellular fluids. Such vapor would be rapidly absorbed by the surrounding fluid environment and might not contribute to any emissions that could be observed by spectrographic analysis of the bubbles of vaporized NaCl. Second, FIG. 1B depicts that the random discrete spark-type events that vaporize tissue are brief—occurring in a the very short, random time intervals in which the insulative bubbles are intact. The lifespan of a bubble is likely to have a duration ranging between 100's of ns to 10's of ms. As is well known, for a tissue ablation to be a substantially cold process, the intervals between successive thermal energy delivery events simply must be longer than the thermal relaxation time of the tissue which is dependent of several tissue characteristics. The thermal relaxation time also must take into consideration the boundary conditions. In the Coblator™ process as depicted in FIG. 1B, the thermal relaxation time of any typical tissue is probably in the range of 100's of ns. In addition, the fluid comprises a boundary condition that acts as a tremendous heat sink. Since the location of spark-type energy applications to tissue is random in location and brief in duration, the combination of (i) the thermal relaxation of the tissue, and (ii) the fluid heat sink removing heat from the tissue can easily return the ablated tissue location to a normal temperature before another random spark-type event occurs in a similar location. Thus, the hypothesis of molecular dissociation is not necessary to explain the “coolness” of the ablation depicted in FIG. 1B.
The types of ablation caused by conventional electrosurgical ablation (see FIG. 1A) and the type of ablation caused by the Coblator™ system (see FIG. 1B)—no matter the hypothesis selected to explain the Coblator™ energy-tissue interaction—share several common characteristics. While conventional and the Coblator™ ablations are suitable for many procedures, both types of ablation are caused by totally random events wherein high electrical energy densities vaporize a fluid or elements therein to create an insulative gas or plasma volume that thereafter causes random localization of an energy-tissue interaction that ablates tissue. By the term random localization, it is meant that while the general location of tissue ablation is known for any time interval, the exact location of an ablation event, for example for any interval in the ns range, is unpredictable. As a further explanation, FIG. 2 represents an enlarged view of a portion of the Coblator™ working end of FIG. 1B with a grid in perspective view as the tissue surface. The location at which the gas volume, or plasma, interfaces the tissue surface is random and the application of energy to the surface will be of very low resolution. If each grid of FIG. 2 is between 10 to 20_m, the lateral distance d between the point of energy emission and the point of highest energy density (herein called the energy deposition or application site) wherein the energy-tissue interaction is localized may as much as 100 to 500_m from a reference axis x of the working end, or the lateral distance d could be zero. A further graphically depiction of what is meant by the concept of random localization of energy-tissue interactions is shown in FIG. 2. In that Figure, the energy delivery horizon (or perimeter) is indicated at h, by which is meant that, at a ns or ms time-scale, the actual application of energy to the tissue may be localized anywhere in the delivery horizon h, and the location of any such application will be entirely random within this horizon.
Another characteristic common to both conventional electrosurgical ablation (FIG. 1A) and the Coblator™ ablation (FIG. 1B) relates to (i) the random size of and energy contact site and (ii) the random distribution of energy across the localized site of energy contact. FIG. 2 graphically depicts a random energy contact and ejecta e from such energy application being irregularly distributed across the energy deposition site. Since the actual energy application occurs only in a dynamic, frothy, expanding and collapsing vapor bubble environment, it is believed that FIG. 2 somewhat accurately depicts the typical energy distribution. In any event, on a ns or ms time-scale, it is clear that the dimensions and energy distribution characteristics of energy delivery are uncontrolled and random. This is to be contrasted with lasers energy delivery systems in which localization can be precise with a few_m's and energy distribution across the site can be designed as Gaussian (higher energy in center of site) or “top-hat” (even energy distribution across the energy deposition site).
While conventional electrosurgical ablation and Coblator™ type ablations are suitable for various procedures, the following characteristics common to both types of ablation prevent the possibility of more precise ablations with such prior art systems:
(i) the actual energy deposition site is randomly localized instead of precisely localized;
(ii) the energy density across the energy deposition site is random and uncontrolled;
(iii) the conductive path between and energy deposition site and the emission point on an electrode is random;
(iv) the dimensions of the energy deposition site are random and uncontrolled;
(v) the amount of energy applied per unit of surface at an energy deposition site is random;
(vi) the amount of energy applied per time interval at an energy deposition site is uncontrolled;
(vii) the duration of intervals between successive energy applications to a site are random;
(viii) the actual duration of an interval of energy application is random and uncontrolled;
(ix) the conductive or insulative characteristics of media between the active electrode and the targeted site, at a micro-scale, are uncontrolled,
(xi) the dimensions, localization, distribution and duration of insulative gas volumes or plasma volumes that facilitate energy delivery are random and uncontrolled; and
(xii) the prior art instrument working ends function dramatically differently depending on the axial distance between an electrode surface and the targeted tissue surface.
It would be highly desirable to have greater precision in thin layer ablations for microsurgeries, neurology, and precision skin resurfacing for burn debridement or cosmetic purposes. What is needed are systems and methods for selective volumetric removal of body surface layer portions (i) that are precisely controllable; (ii) that do not rely on thermal vaporization effects to ablate tissue; (iii) that can be activated in a controlled mode that ratably removes depths or volume of tissue in a given time interval to provide selective tissue decomposition; (iv) that allow for well-defined post-treatment boundaries between layers of tissue decomposition and undamaged layers; (v) that removes surface structure that is exposed to a gas environment or immersed in a fluid environment; (vi) that provide for energy delivery to a patient's body structure via a working end that can be reduced in scale to less than 1 mm. to 3 mm. in diameter for micro-interventional use; and (vii) that utilize a working end that is simple to manufacture and therefore inexpensive and disposable.